Pipe-jacking stoppages modelled using interface shear testing

O’Dwyer, Kevin G.
In recent years, there has been an increased resort to microtunnelling/pipe-jacking as a means of constructing underground conduits (for water, sewage, gas, and other utilities) to avoid on street disruption in urban areas. The difficulty in identifying suitable intermediate shaft locations in built-up areas means that long tunnel drives are often necessary; maintaining jacking forces at manageable levels can be a challenge in these drives. The total jacking force consists of the force at the face of the tunnel boring machine and the frictional force between the pipe string and the surrounding soil, with the latter typically the main contributor to the total jacking force. The introduction of a lubricant into the overcut (the annulus formed on account of the TBM having a larger diameter than the concrete pipes) is well known to be an effective means of reducing skin friction (the frictional force divided by the embedded surface area of the pipe string). While studies have been undertaken to develop a better understanding of the pipe-jacking processes and the factors affecting skin friction, significant gaps remain which are worthy of further research. This study analyses both field and physical modelling results with a primary focus on stoppages, but the impact of buoyancy, bentonite slurry content of material adjacent to the pipeline and shearing rate on the interface shear strength were also examined. Drive data were obtained from 15 pipe-jacking sites in various ground conditions from Ireland, the UK, and Canada. Retrospective analysis of drive data published in the literature has tended to focus on isolated learnings from a single drive/site, whereas a broader view was possible in this study. Once the drives were considered suitable (i.e. screened to check for standard jacking rates, limited amount of excessive deviations from line, etc.), the following conclusions were drawn.  A traditionally used method of separating face resistance and skin friction may be flawed.  There is a general tendency for skin friction to reduce with normalised lubrication ratio (i.e., the lubrication volume ratio (injected bentonite volume normalised by overcut volume) normalised by recommended lubrication volume ratios (Praetorius and Schößer (2017)), but this is complicated by over administration of lubrication in fine soils and under administration in coarse soils.  Pipe buoyancy was demonstrated by estimating the effective normal stress on the pipes as a proportion of the vertical effective stress. Thus, the stability of the overcut could be quantified.  Skin friction increases were noted for stoppages as short as 20 minutes and shown to depend on stoppage duration. Following the literature review and field data analysis, the primary gap identified was quantification of the effect of stoppages on skin friction, which has the potential to increase risk for pipe-jacking contractors, in addition to the underlying mechanism. The effect of bentonite slurry content of the material at the interface (i.e. a proxy for overcut stability) and the shearing rate were considered integral to the stoppages problem and therefore were considered in tandem. Interface shear tests were conducted in both a standard direct shear apparatus and a newly-constructed Interface Shear Facility. The latter facility was designed to incorporate a concrete section cut from a jacking pipe and was capable of shearing at typical field penetration rates. The relationship between shear strength and bentonite slurry content was dependent on whether or not an interface was present. In the absence of an interface, a transition zone was observed where samples were sand-governed up to ≈30% bentonite slurry and clay-governed above ≈80% bentonite slurry. On the other hand, a linear reduction in interface shear strength with bentonite slurry content was observed when the interface was present. The effect of shearing rate on the interface shear strength for a range of sand-bentonite slurry mixtures has not previously been investigated. A more pronounced shearing rate effect was observed for mixes containing greater volumes of bentonite slurry. As shearing rates increased, samples transition from a drained state to an undrained state, which significantly reduced the interface shear strength. To investigate the effect of stoppages on skin friction, a stoppage was incorporated once critical state conditions were reached in these tests, and the increase in shear stress upon recommencement of shearing was noted. These increases were found to be dependent on shearing rate, interface surface roughness and bentonite slurry content at the interface, but there appears to be a threshold stoppage duration beyond which the skin friction increase appears to plateau, suggestive of a time-limited process within the bentonite. Shearometer tests on aged bentonite slurry samples, in addition to consolidation magnitudes during stoppages which are small (relative to pre-shearing magnitudes) and erratic (with time) combine to suggest that bentonite thixotropy is, at least in part, the mechanism behind the stoppage-induced skin friction increases.
NUI Galway
Publisher DOI
Attribution-NonCommercial-NoDerivs 3.0 Ireland